1. Field of the Invention
The present disclosure relates to a semiconductor device.
2. Description of the Related Art
Wide band-gap semiconductors have high breakdown voltage, good electronic transport property, and good thermal conductivity, and thus are very appealing as a material for high temperature, high power, or high frequency semiconductor devices. A typical wide band-gap semiconductor is a nitride-based semiconductor, which is made of GaN, AlN, InN, BN, or a mixed crystal of at least two of GaN, AlN, InN, and BN. Further, in a semiconductor device that has an AlGaN/GaN hetero-junction structure, for example, two-dimensional electron gas is generated at an hetero-junction interface by the piezoelectric effect. The two-dimensional electron gas has high electron mobility and high carrier density. Therefore, semiconductor devices having such an AlGaN/GaN hetero-junction structure, like Schottky barrier diodes or field effect transistors, for example, have high voltage endurance, low on-resistance, and fast switching speed, and are very suitable for power switching applications.
Further, a device having an AlGaN/GaN hetero-junction structure is disclosed, in which a Schottky electrode rides on a surface protection film that is formed on a surface of a semiconductor layer and formed of an electric insulator, and forms a field plate structure, in order to realize higher voltage endurance (refer to “N. Zhang, U. K. Mishra, “High Breakdown GaN HEMT with Overlapping Gate Structure”, IEEE Electron Device Letters, vol. 21, no. 9, 2000”).
In a semiconductor device having a hetero-junction structure, in order to reduce its on-resistance or suppress occurrence of current collapse, a carrier density of its two-dimensional electron gas is preferably increased. If the carrier density of the two-dimensional electron gas is increased, however, intense electrical field concentration tends to occur in the device when a reverse voltage is applied to the device (e.g., in a Schottky barrier diode, when a reverse voltage is applied between an anode electrode and a cathode electrode). This causes reduction of voltage endurance and current collapse of the device.
Accordingly, there is a need to provide a semiconductor device in which reduction of on-resistance or suppression of current collapse is achieved and reduction of voltage endurance is prevented.